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Pension and other post-retirement benefits - Amounts Recognized in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2021 | Dec. 31, 2020
Pension benefits |  |
Actuarial losses (gains) |  |
Balance, January 1 | $ 57,231 | $ 38,510
Additions to AOCI | (59,754) | 50,026
Amortization in current period | (13,130) | (5,430)
Amortization pursuant to plan settlements | (210) |
Reclassification to regulatory accounts | 31,670 | (25,875)
Ending balance, December 31 | 15,807 | 57,231
Past service gains |  |
Beginning balance, January 1 | (5,306) | (6,180)
Additions to AOCI | 237 | (191)
Amortization in current period | 1,626 | 1,609
Amortization pursuant to plan settlements | 0 |
Reclassification to regulatory accounts | (752) | (544)
Ending balance, December 31 | (4,195) | (5,306)
OPEB |  |
Actuarial losses (gains) |  |
Balance, January 1 | (4,299) | (9,146)
Additions to AOCI | (24,126) | 22,036
Amortization in current period | (2,021) | (509)
Amortization pursuant to plan settlements | 0 |
Reclassification to regulatory accounts | 14,816 | (16,680)
Ending balance, December 31 | (15,630) | (4,299)
Past service gains |  |
Beginning balance, January 1 | 0 | 0
Additions to AOCI | 24 | 0
Amortization in current period | 310 | 0
Amortization pursuant to plan settlements | 0 |
Reclassification to regulatory accounts | 0 | 0
Ending balance, December 31 | $ 334 | $ 0

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- Definition Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=124509347&loc=SL7669686-108580 + Details
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- Definition Amount, before tax, of gain (loss) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=124509347&loc=SL7669646-108580 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=124509347&loc=d3e637-108580 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920 + Details
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